WO2007018506A1

WO2007018506A1 - Low cost expansion of capacity for ethylene recovery

Info

Publication number: WO2007018506A1
Application number: PCT/US2005/026661
Authority: WO
Inventors: Rian Reyneke; Michael J. Floral; Guang-Chung Lee; Wayne W. Y. Eng; Iain Sinclair; Jeffrey S. Logsdon
Original assignee: Innovene USA LLC
Current assignee: Ineos USA LLC
Priority date: 2005-07-28
Filing date: 2005-07-28
Publication date: 2007-02-15
Anticipated expiration: 2008-01-28

Abstract

A process for recovery and purification of ethylene and optionally propylene wherein existing olefin plant apparatus is utilized thereby providing significant expansion of the plant’s production capacity with limited additional capital investment, but without increase of energy consumption per unit of desired products. Said process being based on the distributed distillation concept.

Description

LOW COST EXPANSION OF CAPACITY FOR ETHYLENE RECOVERY

The present invention relates to improvements in production facilities for recovery of olefins from suitable sources of mixed hydrocarbon compounds, such as gaseous mixtures from steam or catalytic cracking of petroleum derived streams. More particularly the invention relates to modifying existing, commercial olefin production plants whereby significant expansion of the plant's production capacity is obtained with limited additional capital investment, and without increase in energy consumption per unit of desired olefin products.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under United States Department of Energy Cooperative Agreement No. DE-FC07-01 ID 14090.

. BACKGROUND OF THE INVENTION

Olefin-paraffin separations represent a class of most important and also most costly separations in the chemical and petrochemical industry. Cryogenic distillation has been used for over 60 years for these separations. They remain to be the most energy-intensive distillations because of the close relative volatilities. For example, ethane-ethylene separation is carried out at about negative 25°C and 320 pounds per square inch gauge pressure (psig) in a column containing over 100 trays, and propane-propylene separation is performed by an equally energy-intensive distillation at about negative 3O⁰C and 30 psig.

Impurity refers to compounds that are present in the olefin plant feedstocks and products. Well-defined target levels exist for impurities. Common impurities in ethylene and propylene include: acetylene, methyl acetylene, methane, ethane, propane, propadiene, and carbon dioxide. Listed below are the mole weight and atmospheric boiling points for the light products from thermal cracking and some common compounds potentially found in an olefins unit. Included are some compounds which have similar boiling temperatures to cracked products and may be present in feedstocks or produced in trace amounts during thermal cracking.

Mole Normal Boiling

Compound Weight Point, ⁰C

Hydrogen 2.016 -252.8

Nitrogen 28.013 -195.8

Carbon monoxide 28.010 -191.5

Oxygen 31.999 -183.0 Methane 16.043 -161.5

Ethylene 28.054 -103.8

Ethane 30.070 -88.7

Phosphine 33.970 -87.4

Acetylene * 26.038 -84.0

Carbon dioxide * 44.010 -78.5

Radon 222.00 -61.8

Hydrogen sulfide 34.080 -60.4

Arsine 77.910 -55.0

CCaarrbboonnyyll ssuullffiiddee 6600..007700 -50.3

Propylene 42.081 -47.8

Propane 44.097 -42.1

Propadiene (PD) 40.065 -34.5

Cyclo-propane 42.081 -32.8

MMeetthhyyll aacceettyylleennee 4400..006655 -23.2

Water 18.015 100. * Sublimation temperature

Recently the trend in the hydrocarbon processing industry is to reduce commercially acceptable levels of impurities in major olefin product streams, i.e., ethylene, propylene, and hydrogen. Need for purity improvement is directly related to increasing use of higher activity catalysts for production of polyethylene and polypropylene, and to a limited extent other olefin derivatives.

Processes comprising fractionation steps, as in distillation columns, for the recovery of ethylene from, for example cracked gas origination form ethane are well known. Typically such production of ethylene typically takes place via steam or catalytic cracking of hydrocarbon fractions. This cracking operation is typically non-selective, and produces a reactor effluent that contains, along with ethylene, a number of byproducts as well as unconverted hydrocarbon feed. The byproducts typically produced include methane, hydrogen, acetylene, and other heavier compounds. It is necessary to at least partially separate the ethylene from these byproducts and unconverted hydrocarbons in order to utilize the ethylene in the production of derivative chemical products, such as polyethylene production.

Such ethylene purification is typically done via cryogenic distillation. The hot reactor effluent gas containing the ethylene and various byproducts is generally compressed and cooled before entering the separation equipment.

Many different process designs have been suggested for the separation and purification of ethylene from the reactor effluent gas. A summary of such designs which are currently available has been published recently (Hydrocarbon Processing, March 2003, pp. 96-98). These designs have typically been grouped in terms of the initial rectification operation that is performed. For example, in a "front end demethanizer" design, the compressed, cooled cracked gas is directed to a demethanizer feed chilling train in which it is successively chilled and partially condensed. The condensed liquids are separated and directed to a demethanizer tower. In this tower the methane and lighter components are separated from the ethylene and heavier components. The ethylene and heavier components are typically sent to a deethanizer column where any components heavier than ethane are removed. The resulting deethanizer overhead stream contains primarily ethylene, ethane and acetylene and is directed to an acetylene removal step, such as a hydrogenation reactor. The essentially acetylene-free stream is then directed to a C2 splitter column where the ethylene and ethane are separated. Front end demethanizer designs are often used when the ethylene is produced from a relatively heavy feedstock, such as a naphtha or gas oil.

In a "front end deethanizer" design, the compressed, cooled reactor effluent is directed to a deethanzier column. This column separates the ethane, ethylene, acetylene, and components lighter than ethylene into the deethanizer overhead stream. This stream can be directed to an acetylene removal step, such as a hydrogenation reactor in which the acetylene is removed to a very low level, typically around 1 to 5 ppmw. The low-acetylene stream is then typically directed to a demethanizer via a demethanizer chill train as described above. The bottoms of the demethanizer contains ethylene and ethane and little if any components lighter than ethylene. This bottoms stream is directed to a C2 splitter for recovery of purified ethylene and an ethane recycle stream. Front-end deethanizer designs are often used when the ethylene is produced from a relatively light feedstock, such as ethane, propane, or other relatively light gases.

It should be noted that all ethylene plant designs typically contain at least a demethanizer column, a deethanizer column, and a C2 splitter column. As noted above, the purpose of the demethanizer column is to separate methane and lighter components from the ethylene and any heavier components which are present. Typically the overhead stream from the demethanizer contains less than 1 mol percent ethylene and heavier components, usually less than 0.1 mol percent, The bottoms stream from the demethanizer typically contains a low level of methane and lighter components, typically below 0.1 mol percent, so that ethylene product derived from it is within sales specifications for these components.

The purpose of the deethanizer column is to separate ethane and lighter components from propylene and heavier components. Typically the overhead stream from the deethanizer contains less than 1 mol percent propylene and heavier components, usually less than 0.5 mol percent, and the bottoms stream from the deethanizer contains less than 1 mol percent ethylene and lighter components, usually less than 0.5 mol percent. Finally, the purpose of the C2 splitter column is to separate ethylene from ethane. Typically the overhead product of the C2 splitter column is final purified ethylene product from the ethylene plant and typically has less than 500 ppm ethane. The bottoms stream of the C2 splitter column typically contains primarily ethane with less than about 1 mol percent ethylene.

In addition to the different process designs that have been envisioned for ethylene recovery, there are a number of different types of refrigeration systems that can be used. Historically ethylene recovery designs have utilized a cascaded refrigeration system employing both propylene and ethylene refrigeration loops.

Some designs employ a cascaded methane refrigeration loop as well. Recently a number of designs have been proposed which utilize a binary or ternary mixed refrigerant system. Such systems are beginning to find commercial acceptance within the olefins industry. Mixed refrigerant systems are already well accepted within the natural gas liquids and liquefied natural gas industries.

The use of an ethylene distributor in commercial ethylene manufacture has been described in the literature. US Patent Number 5,675,054 in the name of David Manley and Hazem Hddad describes the use of an ethylene distributor within a more complete distributed distillation flowsheet for a number of cracker feed cases from relatively light to relatively heavy feeds. A similar flowsheet is described by Manley et al. elsewhere ("Optimizing Ethylene Recovery"; Manley and Hahesy, Hydrocarbon Processing, April 1999, p 117). In these flowsheets the ethylene distributor produces an overhead stream containing ethylene and lighter components. This overhead stream is then purified in a demethanizer column to produce a purified ethylene product stream from the bottom of the demethanizer.

US Patent Number 6,212,905 in the name of Keith H. Kuechler and David R. Lumgair also describes an ethylene distributor as used in the production of ethylene-rich product streams. This use is different from that of Manley et al. in that in Kuechler et al. an ethylene product is taken directly from the top of the ethylene distributor without further purification. The ethylene product in Kuechler et al, therefore, also contains light components such as methane and hydrogen. Kuechler et al. further specifies that the overhead stream from the ethylene distributor be produced at a temperature above minus 55⁰F. The Manley literature and Kuechler et al. describe the use of an ethylene distributor in the context of an entire ethylene purification flowsheet. They do not address the possibility of using an ethylene distributor to provide a unique high- capacity expansion option for an existing ethylene production facility.

There remains, therefore, a current need for methods of expansion which demonstrate, through limited modifications of existing commercial ethylene production plants, resulting facilities that allow significant increase in ethylene production, at least about 30 percent, typically at least about 50 percent, more beneficially an increase of at least about 70 percent and possibly even more than 100 percent. Generally, expansions of such magnitude would require the purchase of new and relatively expensive units, such as demethanizer, deethanizer and/or C2 splitter columns.

Particularly advantageous methods of expansion should demonstrate, in addition to such capital cost benefits, that the expanded facilities require less energy per unit of product as compared to facilities enlargement and/or duplication of conventional plants. A lower energy requirement should provide ongoing variable cost savings for the plant expanded.

SUMMARY OF THE INVENTION

The present invention provides means for modification of existing olefin production plants to provide significant expansion of the plant's production capacity with limited additional capital investment, but without increase of energy consumption per unit of desired products. The expanded facility includes a distributive battery comprising separation units to separate, from the gaseous mixture, a primary effluent of methane-free ethylene and a secondary effluent of ethane-free ethylene. Use of independent fractionation groups, using existing apparatus with or without modification, for purification of each olefin-containing effluent from the distributive battery, provides unique high-capacity expansion options for existing commercial ethylene production plants.

In broad aspect, the invention is a method that adapts an existing olefins production facility for recovery of at least a purified ethylene product from a gaseous feed mixture thereby providing an expanded olefins production facility, where the existing olefins production facility comprises: a demethanizer group which during operation exhibits the ability to separate from a gaseous feed mixture, comprising methane, ethylene, ethane, and optionally components heavier than ethane, an overhead stream comprising components in the mixed gas stream which are lighter than ethylene, and a bottoms stream that comprises ethylene and ethane; and a distillation group which during operation exhibits the ability to separate from the stream comprising ethylene and ethane a purified ethylene product essentially free of ethane; and the method of the invention comprises: providing a distributive battery comprising separation units which during operation exhibit the ability to separate, from the gaseous feed mixture, a primary effluent stream, comprising about 15 to 90 percent of the ethylene in the feed mixture and methane, essentially free of ethane; and one or more secondary streams, comprising about 10 to 85 percent of the ethylene in the feed mixture and ethane, essentially free of methane; providing a first fractionation group which during operation exhibits the ability to separate from the stream comprising ethylene and methane a first purified ethylene product essentially free of methane, wherein the first fractionation group comprises one or more substantial element of the existing demethanizer group; and a second fractionation group which during operation exhibits the ability to separate from at least one of the secondary streams comprising ethylene and ethane, a second purified ethylene product essentially free of ethane, wherein the second fractionation group comprises one or more substantial elements of the existing distillation group.

The distributive battery beneficially comprises a fractional distillation unit that during operation exhibits the ability to produce an overhead stream comprising methane, and ethylene, and a bottoms stream comprising ethylene and ethane essentially free of methane. Furthermore, the first fractionation group comprises an existing demethanizer column that during operation exhibits the ability to produce, as a bottoms stream, the first purified ethylene product, and the second fractionation group comprises an existing C2 splitter column that during operation exhibits the ability to produce the second purified ethylene product from an upper section of the existing C2 splitter column, for best results. Typically the method of the invention results in expanded olefin production facilities wherein the total mass flow of purified ethylene products is at least 30 percent greater than the mass flow of the purified ethylene product of the existing olefins production facility.

In another aspect, the invention is an improved method for increasing the ethylene production rate of an existing olefins production facility to provide an expanded olefins production facility, where the existing olefins production facility comprising the steps: directing a mixed gas stream that comprises methane, hydrogen, ethylene, ethane, and components heavier than ethane, but is essentially free of water and carbon dioxide, into an existing integrated chill train and demethanizer column, the demethanizer column producing an overhead stream comprising components in the mixed gas stream which are lighter than ethylene, and a bottoms stream that comprises ethylene and ethane; directing at least a portion of the demethanizer bottoms stream to a deethanizer column, the deethanizer column producing an overhead stream comprising ethylene and ethane and a bottoms stream comprising components heavier than ethane; and directing at least a portion of the deethanizer column overhead directly or indirectly into a splitter column, the splitter column producing an overhead stream which is the purified ethylene product of the existing olefins production facility, and a bottoms stream comprising primarily ethane: and the improvement comprises of the steps: providing an ethylene distributor column, a C2s distributor column, and a front-end rectification column; directing the mixed gas stream to the front- end rectification column and withdrawing therefrom an overhead stream comprising hydrogen, methane, ethylene, ethane, acetylene, and optionally components heavier than ethane, and a bottoms stream comprising components heavier than ethane and essentially free of ethylene; directing the front-end rectification column overhead stream into an acetylene hydrogenation step in which a major fraction of the acetylene is converted to ethylene, ethane, or other hydrogenation products to produce a low-acetylene stream; directing at least a portion of the low-acetylene stream into the C2s distributor column, to produce a C2s distributor overheads stream comprising hydrogen, methane, ethylene, and ethane and a C2s distributor bottoms stream comprising ethylene, ethane, and components heavier than ethane; directing the C2s distributor overheads stream into an ethylene distributor column, to produce an ethylene distributor overheads stream comprising hydrogen, methane and ethylene and essentially free of ethane, and an ethylene distributor bottoms stream comprising ethylene and ethane; directing at least a portion of the ethylene distributor overhead stream into the existing integrated chill train and demethanizer column, to produce a demethanizer overhead stream comprising hydrogen and methane and essentially free of ethylene, and a demethanizer bottoms stream comprising purified ethylene product; directing at least a portion of the ethylene distributor bottoms steam to the existing splitter column; directing at least a portion of the C2s distributor bottoms stream to the existing deethanizer column to produce a deethanizer overhead stream comprising ethylene and ethane: directing the deethanizer overhead stream to the existing splitter column; and withdrawing two purified ethylene products, one from the bottom portion of the existing demethanizer column and another from the upper portion of the existing splitter column, wherein the total mass flow of the two purified ethylene products is greater than the mass flow of the purified ethylene product of the existing olefins production facility.

In one aspect of the invention, at least a portion of the reflux liquid for the C2s distributor column is provided by a liquid side draw from the ethylene distributor column; and optionally the C2s distributor column and the ethylene distributor column are combined into a single divided wall column for best results. In another aspect, within the expanded olefins production facility at least a portion of the reflux liquid for the existing deethanizer column is provided by a liquid side draw from the existing splitter column.

In another aspect the method for increasing the ethylene production rate of an existing olefins production facility the invention provides expanded olefins production facilities, using existing olefins production facility comprising the steps: directing a mixed gas stream that comprises methane, hydrogen, ethylene, ethane, acetylene, and components heavier than ethane, but is essentially free of water and carbon dioxide into a deethanizer column, the deethanizer column producing an overhead stream comprising hydrogen, methane, ethylene, acetylene, and ethane, and a bottoms stream that comprises components heavier than ethane; directing at least a portion of the deethanizer overhead stream into an integrated chill train and demethanizer column, to obtain an overhead stream comprising hydrogen, methane essentially free of ethylene, and a bottoms stream comprising ethylene and ethane from the demethanizer column; and directing at least a portion of the demethanizer column bottoms stream to a splitter column to obtain an overhead stream of purified ethylene product, and a bottoms stream comprising primarily ethane, and steps of improvement consisting of: directing the overhead stream from the existing deethanizer column to an acetylene hydrogenation step to obtain a stream having a low content of acetylene; providing an ethylene distributor column; directing the stream having a low content of acetylene into the ethylene distributor column, and withdrawing therefrom an ethylene distributor overhead stream comprising hydrogen, methane and ethylene and essentially free of ethane, and an ethylene distributor bottoms stream comprising ethylene and ethane; directing a major fraction of the ethylene distributor overhead stream into the existing integrated chill train and demethanizer column and withdrawing therefrom a demethanizer overhead stream comprising hydrogen and methane and essentially free of ethylene, and a demethanizer bottoms stream comprising ethylene; directing at least a portion of the stream having a low content of acetylene into the existing splitter column; and withdrawing two purified ethylene products, one from the bottom portion of the existing demethanizer column and another from the upper portion of the existing splitter column.

In another broad aspect, the invention provides an improved olefins production facility for recovery of at least a purified ethylene product from a gaseous feed mixture comprising methane, ethylene, ethane, propylene, and components heavier than propylene, which facility comprises: (a) a distributive battery comprising separation units which during operation exhibit the ability to separate, from the gaseous mixture a primary effluent stream, comprising about 15 to 90 percent of the ethylene in the feed mixture and methane, essentially free of ethane; and one or more secondary streams, comprising about 10 to 85 percent of the ethylene in the feed mixture and ethane, essentially free of methane; (b) a first fractionation group which during operation exhibits the ability to separate from the stream comprising ethylene and methane a first purified ethylene product essentially free of methane; (c) a second fractionation group which during operation exhibits the ability to separate from one or more of the streams comprising ethylene and ethane a second purified ethylene product essentially free of ethane.

For best results, in an improved olefins production facility according to invention, the distributive battery of separation units comprises: a first distillation unit which during operation exhibits the ability to separate from the gaseous mixture a first overhead stream, comprising methane, ethylene, ethane, and propylene, and a first bottom stream, comprising components heavier than propylene; a second distillation unit which during operation exhibits the ability to separate from the first overhead stream a second overhead stream, comprising ethane and ethylene essentially free of propylene; and second bottom stream, comprising propylene and ethylene, essentially free of methane; a third distillation unit which during operation exhibits the ability to separate from the second overhead stream a third overhead stream, comprising ethylene, essentially free of ethane, and a third bottom stream, comprising ethylene, essentially free of methane; and a fourth distillation unit which during operation exhibits the ability to separate from the second bottom stream a fourth overhead stream, comprising ethylene and ethane, essentially free of propylene, and a fourth bottom stream, comprising propylene, essentially free of ethane.

In one embodiment of the invention, the first fractionation group beneficially comprises an integrated chill train and distillation unit which during operation exhibit the ability to separate from the primary effluent stream comprising ethylene and methane, a first purified ethylene product essentially free of methane. In another embodiment of the invention, the distributive battery of separation units comprises: (a) a distillation unit which during operation exhibits the ability to separate, from the gaseous mixture, an overhead stream, comprising methane, ethylene and ethane, essentially free of propylene; and a subsequent distillation unit which during operation exhibits the ability to separate from the propylene-free overhead stream of (a), another overhead stream, comprising about 15 to 90 percent of the ethylene in the propylene-free overhead stream, essentially free of ethane, and a bottom stream, comprising about 10 to 85 percent of the ethylene, essentially free of methane.

GENERAL DESCRIPTION

According to the invention, the expansion method of this invention applies a distributed distillation sequence to an existing conventional ethylene plant separation section, to permit a significant capacity revamp, while maximizing use of existing equipment. The revised separation sequence requires the addition of new columns to permit non sharp or distributed separation of at least C2 components. A key feature is the production of product quality ethylene from the bottoms of the existing demethanizer and from the overheads of the existing ethylene splitter. The scheme is applicable to most ethylene separation sequences that are currently practiced - though some variation in design detail is required. Existing columns are reused without significant modification. The refrigeration systems will also require expansion, though the invention highlights the use of a mixed refrigeration system to replace the existing ethylene refrigeration system and to work in conjunction with an expanded propylene refrigeration system.

This invention also relates to the expansion of existing commercial ethylene plants. Historically the global demand for ethylene has been increasing. In such an environment, there are two options for an ethylene producer to increase their ethylene production rate. The first is to build a new, grass-roots ethylene production facility. Typically such grass-roots ethylene plants must be quite large, producing around 1000 kilotons of ethylene per year, to provide the economies of scale necessary to produce ethylene at a cost which is competitive in the marketplace. Such plants require very large capital outlays to build, and relatively few are built each year. A second option for producing additional ethylene is to expand an existing commercial ethylene plant

By "expand" in this context we mean to modify an existing commercial ethylene plant so that the ethylene production rate of the modified plant is significantly (at least 30 percent) higher than its previous, unmodified ethylene production rate. Such expansions are common in the ethylene industry. They provide less ethylene than building a new grass-roots ethylene plant, but require much smaller capital outlays and can therefore be more economical than building grass-roots plants.

In addition to adding ethylene furnace capacity, such an expansion is accomplished by modifying specific sections of the plant which would otherwise limit the amount of ethylene which could be produced. For example, certain columns may be fitted with new, specially-designed high-capacity trays to increase the throughput of the column. Alternately, additional columns such as pre-strippers will be installed to provide extra distillation capacity to the separation section. For example, a demethanizer pre-stripper may be installed which partially separates ethylene and heavier materials from the feed stream to the existing demethanizer column. Additionally, it may be necessary to modify the existing process or refrigeration compressors in order to increase the amount of gas they can compress. These and other such modifications are very well known to those skilled in the art of commercial ethylene manufacture and the design and construction of commercial ethylene manufacturing facilities.

This invention relates to the use of distributed distillation for the purpose of achieving an ethylene plant capacity expansion and in particular the use of an ethylene distributor and optionally a C2s distributor. The use of distributed distillation is not new; for many years it has been suggested as a basis for the design of refinery systems, ethylene recovery systems, and other commercial chemical, petroleum and petrochemical separations. Distributed distillation is best understood by contrasting it with sharp split distillation. In sharp split distillation, a separation is made between light and heavy components that are adjacent to each other on the volatility curve of the mixture being separated. That is, there are little or no compounds in the mixture that have volatility that is intermediate to those of the light and heavy components.

For example, as discussed herein above, a typical sharp split deethanizer column in an ethylene recovery system performs a sharp split between ethane and propylene. The overheads of the column contain essentially no propylene and the bottoms contain essentially no ethane. The overheads therefore contain all components lighter than the light component (e.g. ethylene, methane, etc.), and the bottoms contain all components heavier than the heavy component (e.g. propane, C4+, etc.). in a distributed distillation operation, a sharp split is not made between components that are adjacent on the volatility curve. A distributed distillation analog to the deethanizer is a "C2 distributor". A C2 distributor column produces a sharp split between methane and C3 components while distributing ethane and ethylene between the column overhead and bottoms. In a C2 distributor column, the light component is methane and the heavy component is propylene. These components are not adjacent to each other on the volatility curve; ethane and ethylene have a volatility that is intermediate between methane and propylene. In this case, then, ethane and ethylene "distribute" between the column overheads and bottoms. The overheads contain some ethane and ethylene, as well as methane and lighter components, but essentially no propylene. The bottoms also contain some ethane and ethylene, as well as propylene and heavier components, but essentially no methane. Of course, further purification of the components can be done in downstream columns.

Likewise, an ethylene distributor is a distillation column in which a separation is made between methane and ethane. The overhead product of an ethylene distributor contains essentially no ethane and the bottoms product contains essentially no methane. Ethylene, which is intermediate in volatility between methane and ethane, distributes between the top and bottom products.

A benefit of a distributed distillation system is that it requires less total energy to produce the final purified components than an analogous "sharp split" distillation sequence. A way of understanding the energy savings provided by distributed distillation is that it accomplishes the separation of components with fewer overall phase changes. Phase changes (condensation or vaporization) require energy, and reducing the number of phase changes also reduces the energy consumption of the system.

For a more complete understanding of the present invention, reference should now be made to the embodiments illustrated in greater detail in the accompanying drawings and described below by way of examples of the invention.

DESCRIPTION OF THE DRAWINGS

The appended claims set forth those novel features which characterize the present invention. The present invention, as well as advantages thereof, may best be understood by reference to the following brief description of preferred embodiments taken in conjunction with the annexed drawings, in which: FIGURE 1 is a schematic diagram of an ethylene recovery and purification facility used commercially for a source of olefins from a naphtha cracker. (It is a typical "front end demethanizer" design.)

FIGURE 2 is a schematic diagram of an ethylene recovery and purification facility used commercially for a source of olefins from an ethane gas cracker. (It is a typical "front end deethanizer" design.)

FIGURE 3 is a schematic diagram of an olefin recovery and purification facility using an embodiment of the present invention, showing an additional distributive battery to provide ethylene containing feed streams for two different fractionation groups. (The first fractionation group comprises substantial elements derived from the existing demethanizer group; and the second fractionation group comprises one or more elements derived from the existing distillation group shown in FIGURE 1.)

FIGURE 4 is a schematic diagram of an olefin recovery and purification facility using an embodiment of the present invention, showing an additional distributive battery which provides ethylene containing feed streams for two different fractionation groups. (The first fractionation group advantageously comprises substantial elements derived from an existing demethanizer group; and the second fractionation group comprises one or more elements derived from an existing C2 splitter distillation group shown in FIGURE 2.)

In the drawings, like characters designate like or corresponding parts throughout the several views. Auxiliary valves, lines and equipment not necessary for an understanding of the invention have been omitted from the drawings.

The drawings contain illustrative depictions of the concepts of this invention. All major separation steps have been shown. Some details of the process design that are well know to those skilled in the art, such as required heating or cooling steps, vapor-liquid separation drums, process control valves, pumps and the like have been omitted from the drawing in order to demonstrate more clearly the key concepts of the invention. In the FIGURES, columns are shown schematically without the required reboilers, condensers, reflux drums and the like. It is understood that unless otherwise noted the columns in the FIGURES include all such standard equipment necessary for operation of a typical distillation column. EMBODIMENTS OF THE INVENTION

This invention will be described in terms of two separate embodiments, each of which demonstrates how the method of this invention is used to expand the ethylene output of a common generic ethylene plant design. There are numerous distinct ethylene plant designs to which the method of this invention could be applied. The two embodiments described here will demonstrate the basic concepts of the method of this invention, and allow those skilled in the art of ethylene plant design to apply these basic concepts to other ethylene plant designs which are not specifically addressed herein.

Referring to FIGURE 1 , ethylene recovery and purification section 40 depicts, as a schematic diagram, a typical "front end demethanizer" separation design in general use for gases from a naphtha cracker. Some of the process steps in this FIGURE are shown as simple boxes because they admit to a variety of detailed design choices, and the detailed design of these sections does not impact the concept of this invention.

Feed stream 1, comprising naphtha from a suitable source (not shown); enters naphtha cracker unit 2, were it is mixed with steam and directed into the cracking furnaces to produce a hot cracked gas which contains, among other components, hydrogen, methane, ethylene, acetylene, ethane, and heavier hydrocarbons, including unconverted feed. The cracked gas from the furnaces is at high temperature, typically around 1400-1600° F., and relatively low pressure, typically between 15-45 psia. The hot cracked gas is cooled in a quench section. Within this section a number of cooling operations can occur, such as indirect heat exchange with boiler feed water to produce high pressure steam, direct contact with cooled liquid hydrocarbons, direct contact with cooled water, and indirect heat exchange with ambient and sub-ambient cooling media. This quenching action typically results in at least the partial condensation of the cracked gas stream and liquids, both relatively heavy hydrocarbon and water, are typically formed. These liquids can be further processed as desired. The cooled cracked gas stream is compressed to a higher pressure, typically between 200 and 600 psia. Some impurities such as acid gases and water are removed, and the cleaned compressed gas is typically cooled to around ambient temperature to produce the compressed cooled cracked gas stream 3.

Stream 3 enters a demethanizer feed chill train 10, where the cracked gas is chilled to successively lower temperatures against successively colder refrigerant and/or process streams. The successive chilling steps lead to partial condensation of the gas, and after each chilling step the vapor and liquid are typically separated. Vapor enters the next stage of chilling, and liquid exits the chill train as a demethanizer feed stream 11. As noted, there will typically be multiple chilling and vapor/liquid separation steps, so multiple liquid demethanizer feed streams, typically between 2 and 5, are generated. For clarity, only one such stream is shown in FIGURE 1 and it is understood to include the possibility of more liquid demethanizer feed streams. The chill train and cold section of the plant typically also produces a purified hydrogen stream 12 and at least one methane-containing fuel stream 13.

As noted above, some of the chilling of the cracked gas in chill train 10 can be provided by sub-ambient temperature process streams. For example, in FIGURE 1 a cold expanded demethanizer overhead stream, described below, enters the chill train as stream 17 where it is warmed by indirect heat exchange with at least a portion of the cracked gas to be chilled. Typically this warmed expanded demethanizer overhead stream exits the chill train 10 in fuel stream 13.

Liquid demethanizer feed stream or streams can be directed to different feed points on demethanizer column 14. The demethanizer column separates components lighter than ethylene from the ethylene and heavier components. The components lighter than ethylene, for example methane and hydrogen, exit in the demethanizer overhead stream 15. Depending on the composition of this stream it is either sent to a hydrogen recovery section, or it is used, as shown, to provide refrigeration in the process. In the embodiment of FIGURE 1 , the demethanizer overhead stream 15 is expanded in expander 16 and directed, as stream 17, into chill train 10.

Bottoms stream 20, from demethanizer column 14, contains ethylene, acetylene, ethane, and components heavier than ethane. Bottoms stream 20 enters deethanizer column 21 wherein C2 components are separated from the C3 and heavier components. Deethanizer bottoms stream 22 contains essentially all of the components heavier than ethane. This stream can be treated in a variety of ways. According to the embodiment of FIGURE 1 , bottoms stream 20 enters depropanizer column 23 which provides an overhead stream 24 containing primarily C3 hydrocarbons. These hydrocarbons are further separated in the C3 splitter column 25 to produce a purified propylene product stream 26 and a propane stream 27 which can be recycled with feed to the furnace section, sold as product, or used as fuel. Depropanizer bottoms stream 28, which contains hydrocarbons heavier than propane, can be directed to further downstream processing, such as debutanization and butadiene recovery, as is weii known to those skilled in the art of ethylene plant process design.

Deethanizer overhead stream 30, primarily containing ethylene, acetylene, and ethane, is directed to an acetylene hydrogenation unit 31, wherein the deethanizer overhead is contacted with hydrogen (from stream 32) and hydrogenation catalyst under suitable conditions, whereby a majority of the acetylene is hydrogenated to products, primarily ethylene and ethane. Outlet stream 33, from unit 31, typically contains less than 5 ppm of acetylene by weight. Outlet stream 33 enters C2 splitter column 34. A purified ethylene product is withdrawn, as stream 35, from the upper section of column 34. Bottoms stream 36 consisting primarily of ethane is also withdrawn from column 34. All or portions of Stream 36 can be recycled as feed to the furnace section in unit 2, sold as product, or used as fuel.

Referring to FIGURE 2, ethylene recovery and purification section 42 depicts, as a schematic diagram, a typical "front end deethanizer" separation design in general use for gases from an ethane cracker. As in FIGURE 1 , some of the process steps in this FIGURE are shown as simple boxes because they admit to a variety of detailed design choices, and the detailed design of these sections does not impact the concept of this invention.

Feed stream 4, comprising ethane from a suitable source (not shown), enters ethane cracker unit 5, were it goes through the furnace, quench, and compression steps similar to the embodiment of FIGURE 1 to produce the compressed cooled cracked gas stream 6. Cracked gas, typically at a pressure of between 100 and 600 psia, flows into deethanizer column 50. In this column the components heavier than ethane are separated into a bottoms stream 51, which can be treated in a number of ways. Because in a typical ethane cracker plant relatively few components heavier than ethane are produced, stream 51 is typically used as plant fuel. Alternatively, products such as propylene could be recovered from stream 51 if desired.

Deethanizer overhead stream 52 contains ethane, acetylene, ethylene and components lighter than ethylene such as methane and hydrogen. It is compressed in compressor 53, and directed to an acetylene hydrogenation unit 54. The compressed deethanizer overhead stream is contacted with a hydrogenation catalyst under conditions suitable to convert the majority of the acetylene to products of hydrogenation, primarily ethylene and ethane. The acetylene hydrogenation unit outlet stream 55 typically contains no more than about 20 ppm of acetylene by weight, typically less than about 10 ppm, and about 5 ppm for best results.

Stream 55 enters a demethanizer feed chill train 60 similar in function to chill train 10 in FIGURE 1. Within this chill train the cracked gas is chilled to successively lower temperatures against successively colder refrigerant or process streams. The successive chilling steps lead to partial condensation of the gas, and after each chilling step the vapor and liquid are typically separated. The vapor can enter the next stage of chilling and the liquid can exit the chill train as a liquid demethanizer feed stream 61. There will typically be multiple chilling and vapor/liquid separation steps, so multiple liquid demethanizer feed streams, typically between 2 and 5, are generated. For clarity, FIGURE 2 shows only one such stream and it is understood to include the possibility of more liquid demethanizer feed streams. The chill train and cold section of the plant typically also produces a purified hydrogen stream, shown as stream 62, and at least one methane-containing fuel stream, shown as fuel stream 63.

As noted above, some of the chilling of the deethanizer overhead in chill train 60 can be provided by sub-ambient temperature process streams. One such example is shown in FIGURE 2. The cold expanded demethanizer overhead stream, described herein under, enters the chill train as stream 67 where it is warmed by indirect heat exchange with the deethanizer overhead to be chilled. Typically this warmed expanded demethanizer overhead stream would exit chill train 60 in the fuel stream 63.

The liquid demethanizer feed stream or streams designated by stream 61 can be directed to different points on the demethanizer column 64. The demethanizer column separates the components lighter than ethylene from the ethylene and heavier components. The components lighter than ethylene, for example methane and hydrogen, exit in the demethanizer overhead stream 65. Depending on the composition of this stream it is either sent to a hydrogen recovery section, or it is used to provide refrigeration to the process. In the embodiment of FIGURE 2 the demethanizer overhead stream is expanded in expander 66 and directed as stream 67 to the chill train 60, where it is warmed by chilling the cracked gas and exits in the fuel stream 63.

Demethanizer bottoms stream 70 contains primarily ethylene and ethane.

This stream feeds C2 splitter column 71. Overhead stream 72 from the C2 splitter is purified ethylene product, and bottoms stream 73 from the C2 splitter contains primarily ethane which is recycled as additional feed to the furnace in ethane cracker unit 5.

The descriptions of the typical front end demethanizer and typical front end deethanizer ethylene recovery flowsheets are given above for illustration only, and as a basis upon which to demonstrate the method of this invention. It is not meant to be an exhaustive discussion of these flowsheets and those skilled in the art are well aware of the various design options which exist within each.

In order to demonstrate the benefits of this invention, the ethylene production capacity of the front-end demethanizer ethylene recovery and purification section of FIGURE 1 was increased using the method of this invention. The resulting expanded plant is depicted as ethylene recovery and purification section 44 in FIGURE 3. Application of the method of this invention according to FIGURE 3 resulted in a 70 percent increase in the amount of ethylene produced, while still using most of the existing columns of the ethylene recovery and purification section of FIGURE 1 , in particular the demethanizer 14, the deethanizer 21 , the C2 splitter 34, and the depropanizer 23. To accomplish this expansion three new separation steps have been added to the flowsheet, as described below.

Referring to FIGURE 3, feed stream 80, comprising naphtha from a suitable source (not shown), enters modified, expanded naphtha cracker unit 81.

In the naphtha cracker of FIGURE 3, the function and operation of the furnace, quench, and compression steps are essentially unchanged from the base cracker of FIGURE 1. Of course the equipment in these sections will have to undergo modification to handle the additional flows which correspond to a 70 percent expansion in ethylene production. For expansions as large as 70 percent, such modifications would typically involve the installation of larger or parallel equipment as is well known to those skilled in the art.

The compressed cooled cracked gas stream 82 enters a new C3s distributor column 100. In this column a sharp split is made between C2 components and C4 components. In particular, column 100 is operated such that there is a low and controlled level of C4s in the overhead stream 101, and a low and controlled level of C2s in the bottoms stream 102. The C3 components, including propylene, propane, methylacetylene and propadiene, distribute between the overheads and bottoms streams from column 100. Overhead stream 101 is compressed in compressor 103 and then directed to a new acetylene hydrogenation unit 104. This acetylene hydrogenation unit replaces the existing acetylene hydrogenation unit 31, OF FIGURE 1. It is beneficial to remove acetylene at this point in the flowsheet so as to avoid having to remove it at multiple downstream locations.

The acetylene hydrogenation reactor effluent stream 105 is typically cooled against ambient and optionally sub-ambient cooling media and directed into a new C2s distributor column 110. In this column a sharp split is made between the

C3 components and components lighter than ethylene. In particular, column 110 is operated such that there is a low and controlled level of C3s in overhead stream 111, and a low and controlled level of components lighter than ethylene, for example methane, in bottoms stream 112. The C2 components, including ethylene, ethane, and any remaining acetylene, distribute between the overheads and bottoms streams of 110.

Bottoms stream 112, containing C2 and C3 hydrocarbons is directed to deethanizer 21 e. A beneficial aspect of this invention is that deethanzier column 21 from the existing olefins production facility of FIGURE 1 is used without substantial modification as deethanizer 21 e in the expanded olefins production facility of FIGURE 3. For the purposes of this invention, using a column "without substantial modification" indicates that for it's use in the expanded plant no additional pressure shells need to be installed and that at least 75 percent of the existing trays in the column can be used "as-is\ i.e., without modification. Changes to the column feed location or locations, or the addition of one or more feed locations is not considered a substantial modification for the purposes of this invention. Likewise, changes to the reboiler and/or condenser exchangers and the various drums and pumps required for column operation are not considered to be substantial modifications for the purposes of this invention.

Deethanizer 21 e produces overhead stream 113, containing C2 hydrocarbons, and bottoms stream 114, containing primarily C3 hydrocarbons. Stream 113 is directed into C2 splitter 34e. It is a further beneficial aspect of this invention that the C2 splitter column 34 from the existing olefins production facility of FIGURE 1 is used, without substantial modification, as C2 splitter 34e in the expanded olefins production facility of FIGURE 3. Bottoms stream 114, flows into C3 splitter 115. The C3 splitter 115 in the expanded olefins production facility of FIGURE 3 is larger than the C3 splitter 25 of the existing olefins production facility of FIGURE 1. Splitter 115 produces an overhead product stream 116 of purified propylene product and a bottoms stream 117 of propane that can be recycled into feed for the furnace section of naphtha cracker unit 81 , sold, or used as fuel. Overhead stream 111 from the C2s distributor column 110 is directed to a new ethylene distributor column 120, wherein a sharp split is made between methane and ethane. In particular, column 120 is operated such that there is a low and controlled level of ethane in overhead stream 121 , and a low and controlled level of methane in bottoms stream 122. Ethylene distributes between the overheads and bottoms streams of 120. Bottoms stream 122, containing ethane and ethylene, is directed to the C2 splitter column 34e. A significant fraction of the ethylene that would have otherwise entered the C2 splitter in the expanded case goes to the ethylene distributor overhead stream 121 , thereby bypassing C2 splitter 34e. Overhead stream 123 is a first purified ethylene product, and bottoms stream 124 contains essentially only ethane. Stream 124 can be recycled with feed into the furnace section of naphtha cracker unit 81 , sold, or used as fuel.

Overhead stream 121 of ethylene distributor column 120 is directed into a demethanizer feed chill train 130 similar in function to chill train 10 of FIGURE 1 , but in this case the feed entering the chill train 130 contains essentially no ethane. It is anticipated that some of the equipment from the existing chill train 10 is reused within the modified chill train 130, while other equipment within chill train 130 will be new.

Within chill train 130, the ethylene distributor overhead is chilled to successively lower temperatures against successively colder refrigerant or process streams. The successive chilling steps lead to partial condensation of the vapor, and after each chilling step the vapor and liquid are separated. The vapor can enter the next stage of chilling and the liquid can exit the chill train as a liquid demethanizer feedstream 131. There will typically be multiple chilling and vapor/liquid separation steps, so multiple liquid demethanizer feed streams, typically between 2 and 5, are generated. For clarity, FIGURE 3 shows only one such stream and it is understood to include the possibility of more liquid demethanizer feed streams. The chill train and cold section of the plant also produces a purified hydrogen stream 132 and at least one methane-containing fuel stream 133.

As noted above, some of the chilling of the ethylene distributor overhead in chill train 130 can be provided by sub-ambient temperature process streams.

Similar to the embodiment of FIGURE 1 , the cold expanded demethanizer overhead stream, described below, enters the chill train as stream 137 where it is warmed by indirect heat exchange with the ethylene distributor overhead stream to be chilled. Typically this warmed, expanded demethanizer overhead stream exits chill train 130 in a fuel stream 133.

The liquid demethanizer feed stream or streams represented by 131 can be directed to different points on the demethanizer column 14e. It is a further beneficial aspect of this invention that demethanzier column 14 from the existing olefins production facility of FIGURE 1 is used, without substantial modification, as demethanizer column 14e in the expanded olefins production facility of FIGURE 3. Components lighter than ethylene, for example methane and hydrogen, exit in the demethanizer overhead stream 135. Depending on the composition of this stream it is either sent to a hydrogen recovery section, or it is used to provide refrigeration to the process. In the embodiment of FIGURE 3 the demethanizer overhead stream is expanded in expander 136 and directed as stream 137 into the chill train, where it is warmed by chilling at least a portion of the ethylene distributor overhead stream and exits section 130 in the fuel stream 133.

Bottoms stream 140 from the demethanizer is a second purified ethylene product, which when combined with the first purified ethylene product stream 123 forms the total purified ethylene product stream 141 from the expanded olefins plant of FIGURE 3.

Finally, bottoms stream 102 from the C3s distributor column 100 feeds depropanizer column 23e. It is a further beneficial aspect of this invention that the depropanizer column 23 from the existing olefins production facility of FIGURE 1 is used, without substantial modification, as depropanizer column 23e in the expanded olefins production facility of FIGURE 3. The depropanizer column overhead stream 142, containing C3 hydrocarbons, enters the C3 splitter column 115. The depropanizer column bottoms stream 143 contains hydrocarbons heavier than propane. This stream can be directed to further downstream processing, such as debutanization and butadiene recovery, as is well known to those skilled in the art of ethylene plant process design.

The ability to use existing columns even with such a large expansion directly results from the method of this invention. In particular it results from the addition of the ethylene distributor and the C2s distributor columns, and their specific placement relative to the existing demethanizer, deethanizer, and C2 splitter in the expanded plant flowsheet. The ethylene distributor column, column 120, does a partial separation of ethylene and ethane, a separation which was carried out solely in the C2 splitter column 34 of the base plant of FIGURE 1. Thus, a portion of the ethylene in the expanded plant is sent overhead of the ethylene distributor and does not enter the C2 splitter. In addition, the demethanizer in the expanded plant is unloaded relative to the base plant because the feed to the demethanizer contains only a portion of the total ethylene flow and no ethane.

The use of the C2s distributor column 110 also partially unloads the existing columns. In the expanded case of FIGURE 3 some of the C2 components are sent overhead of the C2s distributor, so there is relatively less feed entering the existing deethanizer column 21 e than in the base case of FIGURE 1. Finally, in the expanded case there is pre-separation of the C2 splitter column feed. The total feed to the C2 splitter column 34e now enters the column as separate streams at separate points. This allows the C2 splitter to operate more efficiently than when there is no pre-separation and all of the feed enters the column at a single point, as is the case in the base plant of FIGURE 1.

It should be noted that a number of design options exist for the expanded olefins plant of FIGURE 3. For example it may be possible to take advantage of opportunities for thermally coupling pairs of columns in this flowsheet. For the purpose of this discussion, thermally coupled columns are those in which the reflux liquid for one column is provided by a liquid side draw from a downstream column. For example it has been found to be beneficial from an energy and capital standpoint to thermally couple the C2s distributor 110 and the ethylene distributor 120 such that the reflux liquid for the C2s distributor is provided by a liquid side draw from the ethylene distributor. Similarly, reflux liquid for the C3s distributor 100 may be provided by a liquid side draw from the C2s distributor column 110. Finally, reflux liquid for the deethanizer 21 e can be provided by a liquid sidedraw from the C2 splitter 34e. Such thermal coupling of columns is contained within the scope of this invention.

An additional modification can be made if the C2s distributor 110 and ethylene distributor 120 are thermally coupled. It has been recognized that in such a case it may be beneficial to combine the two columns into a single divided wall column which produces a single overhead product stream 121 and two separate bottoms streams 112 and 122. Such a design is disclosed in commonly assigned US Patent Application 10/393,029.

We have found that it can be highly beneficial to utilize a mixed refrigeration system in conjunction with the expanded olefins plant of FIGURE 3.

Conventionally, olefins^' production facilities utilize refrigeration systems based on pure propylene and ethylene working fluids to provide the chilling required for the recovery and purification of ethylene. The use of mixed refrigeration systems in the production of olefins is also practiced, though less extensively than the use of propylene and ethylene-based systems. In a mixed refrigerant system the working fluid is composed of more than one component. For example, the working fluid can be composed of a mixture of hydrocarbons such as methane, ethane, ethylene, propane, propylene, butane, or butene. In a closed-loop mixed refrigerant system, the working fluid is compressed, cooled and at least partially condensed, and then flashed to lower pressure whereupon any remaining liquid is vaporized to provide refrigeration at a temperatures below ambient temperature. The relatively low-pressure vaporized mixed refrigerant working fluid is then recycled to the compressor to be re-compressed.

It should be recognized that the refrigeration system or systems of the base plant of FIGURE 1 would also have to be expanded to provide the increased refrigeration duty required by the expanded plant of this invention, as depicted in FIGURE 3. The expansion of such refrigeration systems can take two forms. For relatively smaller expansions, the compressors can be re-wheeled to provide additional flow and therefore additional refrigeration capacity. For relatively larger expansions, such as expansions as large as those provided by the method of this invention, current practice requires the addition of new parallel propylene, ethylene, and/or mixed refrigerant systems to provide the significantly higher refrigeration requirement of the expanded plant. We have found that the installation of a mixed refrigeration system provides unexpected synergies when combined with the method of this invention.

In particular we have found that installation of a mixed refrigeration system which both replaces the existing low-temperature ethylene refrigeration system and also provides some duty in the propylene refrigeration temperature range, e.g. above about negative 45° F., will allow the expanded plant to be operated without the installation of a new, parallel propylene refrigeration system. By the appropriate selection of mixed refrigerant composition and compressor inlet and outlet pressures a mixed refrigeration system is well suited to provide refrigeration which covers both the ethylene and propylene temperature ranges.

As discussed above, the ethylene distributor column in the expanded plant of FIGURE 3 significantly unloads the C2 splitter in the expanded plant. Utilizing mixed refrigerant to provide at least a part of the ethylene distributor condensing duty allows this portion of the ethylene/ethane separation to be accomplished without the use of low-level propylene refrigeration. Therefore by including a suitably-designed mixed refrigeration system in the expanded plant design, no parallel propylene refrigeration system needs to be installed. This will result in significant capital cost savings for the expansion, and is a direct result of the unique expansion design made possible by the method of this invention.

In order to further demonstrate the benefits of this invention, the ethylene production capacity of the front-end deethanizer ethane cracker of FIGURE 2 was also increased using the method of this invention. Referring to FIGURE 4, ethylene recovery and purification section 46 depicts, as schematic diagram, the resulting expanded embodiment of the invention. Application of the method of this invention according to FIGURE 4 resulted in a 90 percent increase in the amount of ethylene produced by the process, while still using most of the existing columns of the process of FIGURE 2, in particular the deethanizer 50, the demethanizer 64, and the C2 splitter 71.

Feed stream 83, comprising ethane from a suitable source (not shown), enters ethane cracker unit 84, were it goes through the furnace, quench, and compression steps similar to the embodiment of FIGURE 2 to produce the compressed cooled cracked gas stream 85. In the modified, expanded ethane cracker of FIGURE 4, the function and operation of the furnace, quench, and compression steps are essentially unchanged from the base olefins plant of FIGURE 2. Of course the equipment in these sections will have to undergo modification to handle the additional flows which correspond to a 90 percent expansion in ethylene production. For expansions as large as 90 percent, such modifications would typically involve the installation of larger or parallel equipment as is well known to those skilled in the art.

The compressed cooled cracked gas stream 85 enters an expanded deethanizer column 160. It is anticipated that the deethanizer 50 from the existing olefins plant of FIGURE 2 can be re-used in the expanded plant, but that an additional, parallel deethanizer column would have to be installed to handle the increased cracked gas flow. Both the new deethanizer column and the existing deethanizer column 50 are represented by the single deethanizer block 160. The deethanizer produces a bottoms product stream 161 which contains primarily hydrocarbons heavier than ethane. This stream can be sent to further purification to recover chemical products, or used as fuel. The deethanizer overhead stream 162 contains primarily ethane, acetylene, ethylene, and components lighter than ethylene, such as methane and hydrogen. Similar to the existing olefins plant of FIGURE 2, this stream can be compressed in compressor 163 after which it enters an acetylene hydrogenation unit 164. The acetylene hydrogenation unit 164 functions in a manner similar to the acetylene hydrogenation unit 54 of the base plant of FIGURE 2. However, the capacity of the acetylene hydrogenation equipment must be increased to handle the significantly increased flow of the expanded case.

The compressed acetylene-free deethanizer overhead stream 165 enters a new ethylene distributor column 170. In this column a sharp split is made between methane and ethane, in particular, column 170 is operated such that there is a low and controlled level of ethane in the overhead stream 171, and a low and controlled level of methane in the bottoms stream 172. Ethylene distributes between the overheads and bottoms streams of 170. The bottoms stream 172 contains primarily ethane and ethylene and is directed to the C2 splitter 71 e. It is a beneficial aspect of this invention that the C2 splitter column 71 e from the existing olefins production facility of FIGURE 2 can be used without substantial modification in the expanded olefins production facility of FIGURE 4. The overhead product of the C2 splitter is a first purified ethylene stream 173. The bottoms stream 174 from the C2 splitter contains primarily ethane and is typically recycled as feed to the furnace section 84.

The ethylene distributor overhead stream 171 enters a demethanizer feed chill train 180 similar in function to chill train 60 of FIGURE 2, but in this case the feed entering the chill train 180 contains essentially no ethane. It is anticipated that some of the equipment from the existing chill train 60 could be re-used within the modified chill train 180, while other equipment within chill train 180 will be new.

Within this modified chill train the ethylene distributor overhead stream 171 is chilled to successively lower temperatures against successively colder refrigerant or process streams. The successive chilling steps lead to partial condensation of the vapor, and after each chilling step the vapor and liquid are separated. The vapor can enter the next stage of chilling and the liquid can exit the chill train as a liquid demethanizer feed stream 181. There will typically be multiple chilling and vapor/liquid separation steps, so multiple liquid demethanizer feed streams, typically between 2 and 5, are generated. For clarity, FIGURE 4 shows only one such stream and it is understood to include the possibility of more liquid demethanizer feed streams. The chill train and cold section of the plant typically also produces a purified hydrogen stream 182 and at least one methane- containing fuel stream 183. As noted above, some of the chilling of the ethylene distributor overhead in chill train 180 can be provided by sub-ambient temperature process streams. Similar to the embodiment of FIGURE 2, the cold expanded demethanizer overhead stream, described herein under, enters the chill train as stream 187 where it is warmed by indirect heat exchange with the ethylene distributor overhead stream to be chilled. Typically this warmed expanded demethanizer overhead stream would exit chill train 180, in the fuel stream 183.

The liquid demethanizer feed stream or streams represented by 181 can be directed to different points on the demethanizer column 64e. It is a further beneficial aspect of this invention that the demethanzier column 64e from the existing olefins production facility of FIGURE 2 can be used without substantial modification in the expanded olefins production facility of FIGURE 4. The components lighter than ethylene, for example methane and hydrogen, exit in the demethanizer overhead stream 185. Depending on the composition of this stream it is either sent to a hydrogen recovery section, or it is used to provide refrigeration to the process. In the embodiment of FIGURE 4 the demethanizer overhead stream is expanded in expander 186 and directed as stream 187 to the chill train, where it is warmed by chilling at least a portion of the ethylene distributor overhead stream and exits chill train 180 in the fuel stream 183.

The bottoms of the demethanizer, stream 190, consists of a second purified ethylene product which when combined with the first purified ethylene product stream 173 represents the total ethylene product stream 191 from the expanded olefins plant of FIGURE 4.

The following examples will serve to illustrate certain specific embodiments of the invention disclosed herein. These examples should not, however, be construed as limiting the scope of the novel invention, as there are many variations which may be made thereon without departing from the spirit of the disclosed invention, as those of skill in the art will recognize.

EXAMPLES Example 1 : Expansion of An Existing Front-End Demethanizer Plant

The ethylene recovery and purification section 40 depicted in FIGURE 1 and the expanded recovery and purification section 44 in FIGURE 3 were simulated using commercially-available process simulation software. In the simulation the base front-end demethanizer naphtha cracker of FIGURE 1 produces approximately 500,000 metric tones per year of purified ethylene product. The column design parameters, including column diameter and number of trays were determined for this ethylene production capacity. In the simulation of the plant expanded according to the method of this invention, shown in FIGURE 3, the column design parameters for the demethanizer, C2 splitter, deethanizer, and depropanizer were kept constant. The feed rate to the expanded plant was then increased until a capacity limit was reached in one of these four columns. This maximum feed rate determined the maximum ethylene production capacity of the expanded plant. For the case of the expanded plant flowsheet shown in FIGURE 3, the maximum ethylene production capacity of the expanded plant was 72 percent greater than the ethylene production capacity of the base ethylene plant of FIGURE 1.

Table I presents selected stream flow and composition data for the base olefins plant of FIGURE 1 and Table Il shows selected stream flow and composition data for the expanded olefins plant of FIGURE 3. It is apparent that the ethylene production rate of the expanded plant, stream 141 of Table II, is approximately 70 percent greater than the ethylene production rate of the base plant, stream 35 of Table I. Furthermore, examination of the data in Table I and Il shows that the molar feed rate to the demethanizer in the expanded plant, stream 131 in Table II, is actually lower than that of the base plant, stream 11 in Table I. Similarly, the feed to the deethanizer in the expanded plant, stream 112 of Table II, is lower than that of the base plant, stream 20 of Table I. The total feed to the C2 splitter is roughly similar for both the expanded plant, the sum of streams 113 and 122 in Table II, and the base plant, stream 33 of Table I. These surprising effects result from application of the methods of this invention, particularly the installation of the ethylene distributor and the C2s distributor columns 110 and 120.

Table ill presents the design parameters for the demethanizer, C2 splitter, deethanizer, and depropanizer for both the base plant of FIGURE 1 and for the expanded plant of FIGURE 3. The "Percent Capacity" of the base plant columns was set at 100 percent. The "Percent Capacity" values of the corresponding columns in the expanded plant were determined from the percent approach to flood on the limiting trays of the column as determined from the simulation of the expanded plant. It can be seen from Table III that the depropanizer column limits the expansion of the plant of this example. The depropanizer is at 100 percent capacity in the both the base and expanded cases. It is worth noting that the demethanizer, C2 splitter, and deethanizer all have some reserve capacity in the expanded plant, even at 72 percent greater ethylene production than the base plant. This suggests that if the bottleneck in the depropanizer could be overcome (for example through the use of high-capacity trays), an expansion of over 72 percent may be possible. The ability to expand the ethylene production of the base plant to such a large degree, while still using these existing columns, is a result of the unique expansion method of this invention.

In addition to the ability to re-use existing equipment in the expanded case, the expansion brought about by the method of this invention requires less energy than a conventional expansion. For such a large expansion, i.e. about 72 percent, it would be anticipated that a separate, parallel purification train would be required for a conventional expansion. An analysis was made of the energy requirement of the conventional expanded case, i.e. twin parallel trains, and the expansion case of this invention (shown in FIGURE 3). The plant which was expanded using conventional means required approximately 9100 BTU of energy for each pound of ethylene produced, while the plant which was expanded according to the method of this invention required approximately 8550 BTU for each point of ethylene produced. The difference in energy requirement is therefore approximately 550 BTU per pound of ethylene, which provides significant variable cost savings for the plant expanded according to the method of this invention.

Example 2: Expansion of An Existing Front-End Deethanizer Plant

The ethylene recovery and purification section 42 depicted in FIGURE 2 and the expanded recovery and purification section 46 in FIGURE 4 were simulated using commercially-available process simulation software. In the simulation the base front-end deethanizer ethane cracker of FIGURE 2 produces approximately 500,000 metric tones per year of purified ethylene product. The column design parameters, including column diameter and number of trays were determined for this ethylene production capacity. In the simulation of the plant expanded according to the method of this invention, shown in FIGURE 4, the column design parameters for the demethanizer and C2 splitter were kept constant. The feed rate to the expanded plant was then increased until a capacity limit was reached in one of these four columns. This maximum feed rate determined the maximum ethylene production capacity of the expanded plant. For the case of the expanded plant flowsheet shown in FIGURE 4, the maximum ethylene production capacity of the expanded plant was 91 percent greater than the ethylene production capacity of the base ethylene plant of FIGURE 2.

Table IV presents selected stream flow and composition data for the base olefins plant of FIGURE 2 and Table V shows selected stream flow and composition data for the expanded olefins plant of FIGURE 4. It is apparent that the ethylene production rate of the expanded plant, stream 191 of Table V, is approximately 91 percent greater than the ethylene production rate of the base plant, stream 72 of Table IV. Furthermore, the molar feed rate to the demethanizer in the expanded plant, stream 181 in Table V, is actually lower than that of the base plant, stream 61 in Table IV. The total feed to the C2 splitter is slightly higher in the expanded plant, stream 172 in Table V, than in the base plant, stream 70 of Table IV. The existing C2 splitter column can be used even with a slightly higher feed rate because of a more efficient feed system to the column: the total feed is split into two fractions, one is vaporized and sent to a lower point on the column, and the other enters an upper point on the column without modification. These surprising effects result from application of the methods of this invention, particularly the installation of the ethylene distributor column 170.

Table Vl presents the design parameters for the demethanizer and C2 splitter for both the base plant of FIGURE 2 and for the expanded plant of

FIGURE 4. As in the first example, the "Percent Capacity" of the base plant columns was set at 100 percent. The "Percent Capacity" values of the corresponding columns in the expanded plant were determined from the percent approach to flood on the limiting trays of the column as determined from the simulation of the expanded plant. It is clear from Table Vl that the C2 splitter column limits the expansion of the plant of this example: the C2 splitter is at 100 percent capacity in the both the base and expanded plants. The demethanizer still has significant reserve capacity in the expanded plant, even at 91 percent greater ethylene production than the base plant. This suggests that if the bottleneck in the C2 splitter could be overcome (for example through the use of high-capacity trays), an expansion of over 90 percent and perhaps in excess of

100 percent may be possible. The ability to expand the ethylene production of the base plant to such a large degree, while still using these existing columns, is a result of the unique expansion method of this invention.

In addition to the ability to re-use existing equipment in the expanded case, the expansion brought about by the method of this invention requires less energy than a conventional expansion. For such a large expansion of purified ethylene capacity, i.e. 91 percent, it would be anticipated that a separate, parallel purification train would be required for a conventional expansion. An analysis was made of the energy requirement of the conventional expanded case, i.e. twin parallel trains, and the expansion case of this invention (shown in Figure 4). The plant which was expanded using conventional means required approximately 6030 BTU of energy for each pound of ethylene produced, while the plant which was expanded according to the method of this invention required approximately 562 0 BTU for each point of ethylene produced. The difference in energy requirement is therefore approximately 410 BTU per pound of ethylene, which provides significant variable cost savings for the plant expanded according to the method of this invention.

For the purposes of the present invention, "predominantly" is defined as more than about fifty percent. "Substantially" is defined as occurring with sufficient frequency or being present in such proportions as to measurably affect macroscopic properties of an associated compound or system. Where the frequency or proportion for such impact is not clear, substantially is to be regarded as about twenty per cent or more. The term "a feedstock consisting essentially of" is defined as at least 95 percent of the feedstock by volume. The term "essentially free of" is defined as absolutely except that small variations which have no more than a negligible effect on macroscopic qualities and final outcome are permitted, typically up to about one percent.

TABLE I Flows and Conditions for Streams for Base Plant of Figure 1

ω

TABLE Il Flows and Conditions for Streams in Expanded Plant of Figure 3

TABLE III Column Design Summary For Front-End Demethanizer Naphtha Cracker

TABLE IV Flows and Conditions for Streams in Base Plant of Figure 2

CO

TABLE V Flows and Conditions for Streams in Expanded Plant of Figure 4

CO en

TABLE Vl

Column Design Summary For Front-End Deethanizer Ethane Cracker

CO

Claims

That which is claimed is:

1. A method that adapts an existing olefins production facility for recovery of at least a purified ethylene product from a gaseous feed mixture thereby providing an expanded olefins production facility, where the existing olefins production facility comprises:

(1-a) a demethanizer group which during operation exhibits the ability to separate from a gaseous feed mixture, comprising methane, ethylene, ethane, and optionally components heavier than ethane, an overhead stream comprising components in the mixed gas stream which are lighter than ethylene, and a bottoms stream that comprises ethylene and ethane; and

(1-b) a distillation group which during operation exhibits the ability to separate from the stream comprising ethylene and ethane a purified ethylene product essentially free of ethane; and wherein the method comprises:

(i) providing a distributive battery comprising separation units which during operation exhibit the ability to separate, from the gaseous feed mixture, a primary effluent stream, comprising about 15 to 90 percent of the ethylene in the feed mixture and methane, essentially free of ethane; and one or more secondary streams, comprising about 10 to 85 percent of the ethylene in the feed mixture and ethane, essentially free of methane;

(ii)providing a first fractionation group which during operation exhibits the ability to separate from the stream comprising ethylene and methane a first purified ethylene product essentially free of methane, wherein the first fractionation group comprises one or more substantial element of the existing demethanizer group; and

(iii) a second fractionation group which during operation exhibits the ability to separate from at least one of the secondary streams comprising ethylene and ethane, a second purified ethylene product essentially free of ethane, wherein the second fractionation group comprises one or more substantial elements of the existing distillation group.

2. The method of claim 1 wherein the distributive battery comprises a fractional distillation unit that during operation exhibits the ability to produce an overhead stream comprising methane and ethylene, and a bottoms stream comprising ethylene and ethane essentially free of methane;

3. The method of claim 2 wherein the first fractionation group comprises an existing demethanizer column that during operation exhibits the ability to produce, as a bottoms stream, the first purified ethylene product.

4. The method of claim 2 wherein the second fractionation group comprises an existing C2 splitter column that during operation exhibits the ability to produce the second purified ethylene product from an upper section of the existing C2 splitter column.

5. The method of claim 1 wherein the total mass flow of the two purified ethylene products is at least 30 percent greater than the mass flow of the purified ethylene product of the existing olefins production facility.

6. A method for increasing the ethylene production rate of an existing olefins production facility to provide an expanded olefins production facility, the existing olefins production facility comprising the steps:

(6-a) directing a mixed gas stream that comprises methane, hydrogen, ethylene, ethane, acetylene and components heavier than ethane, but is essentially free of water and carbon dioxide, into an integrated chill train and existing demethanizer column, the demethanizer column producing an overhead stream comprising components in the mixed gas stream which are lighter than ethylene, and a bottoms stream that comprises ethylene, ethane and components heavier than ethane;

(6-b) directing at least a portion of the demethanizer bottoms stream to a deethanizer column, the deethanizer column producing an overhead stream comprising ethylene and ethane and a bottoms stream comprising components heavier than ethane; and (6-c) directing at least a portion of the deethanizer column overhead directly or indirectly into a splitter column, the splitter column producing an overhead stream which is the purified ethylene product of the existing olefins production facility, and a bottoms stream comprising primarily ethane: wherein the improvement comprises of the steps:

(i) providing an ethylene distributor column, a C2s distributor column, and a front-end rectification column;

(ii)directing the mixed gas stream to the front-end rectification column and withdrawing therefrom an overhead stream comprising hydrogen, methane, ethylene, ethane, acetylene, and components heavier than ethane, and a bottoms stream comprising components heavier than ethane and essentially free of ethylene; (iii) directing the front-end rectification column overhead stream from (ii) to an acetylene hydrogenation step in which a major fraction of the acetylene is converted to ethylene, ethane, or other hydrogenation products to produce a low-acetylene stream; (iv) directing at least a portion of the low-acetylene stream from (iii) to the C2s distributor column, to produce a C2s distributor overheads stream comprising hydrogen, methane, ethylene, and ethane and a C2s distributor bottoms stream comprising ethylene, ethane, and components heavier than ethane; (v) directing the C2s distributor overheads stream to an ethylene distributor column, to produce an ethylene distributor overheads stream comprising hydrogen, methane and ethylene and essentially free of ethane, and an ethylene distributor bottoms stream comprising ethylene and ethane; (vi) directing at least a portion of the ethylene distributor overhead stream into the integrated chill train and existing demethanizer column, to produce a demethanizer overhead stream comprising hydrogen and methane and essentially free of ethylene, and a demethanizer bottoms stream comprising purified ethylene product; (vii) directing at least a portion of the ethylene distributor bottoms steam to the existing splitter column;

(viii) directing at least a portion of the C2s distributor bottoms stream to the existing deethanizer column to produce a deethanizer overhead stream comprising ethylene and ethane: (ix) directing the deethanizer overhead stream to the existing splitter column; and

(x) withdrawing two purified ethylene products, one from the bottom portion of the existing demethanizer column and another from the upper portion of the existing splitter column, wherein the total mass flow of the i two purified ethylene products is greater than the mass flow of the purified ethylene product of the existing olefins production facility.

7. The method of claim 6 wherein the portion of the mixed gas stream directed into the ethylene distributor column is essentially free of acetylene.

8. The method of claim 6 wherein the total mass flow of the two purified ethylene products is at least 30 percent greater than the mass flow of the purified ethylene product of the existing olefins production facility.

9. The method of claim 6 wherein at least a portion of the reflux liquid for the C2s distributor column is provided by a liquid side draw from the ethylene distributor column.

10. The method of claim 6 wherein the C2s distributor column and the ethylene distributor column are combined into a single divided wall column.

11. The method of claim 6 wherein within the expanded olefins production facility at least a portion of the reflux liquid for the existing deethanizer column is provided by a liquid side draw from the existing splitter column.

12. A method for increasing the ethylene production rate of an existing olefins production facility to provide an expanded olefins production facility, the existing olefins production facility comprising the steps:

(12-a) directing a mixed gas stream that comprises methane, hydrogen, ethylene, ethane, acetylene, and components heavier than ethane, but is essentially free of water and carbon dioxide into a deethanizer column, the deethanizer column producing an overhead stream comprising hydrogen, methane, ethylene, acetylene, and ethane, and a bottoms stream that comprises components heavier than ethane;

(12-b) directing at least a portion of the deethanizer overhead stream into an integrated chill train and demethanizer column, to obtain an overhead stream comprising hydrogen and methane and essentially free of ethylene, and a bottoms stream comprising ethylene and ethane from the demethanizer column; and

(12-c) directing at least a portion of the demethanizer column bottoms stream to a splitter column to obtain an overhead stream of purified ethylene product, and a bottoms stream comprising primarily ethane, wherein the improvement consists of the steps:

(i) directing the overhead stream from the existing deethanizer column to an acetylene hydrogenation step to obtain a stream having a low content of acetylene;

(ii)providing an ethylene distributor column;

(iii) directing the stream having a low content of acetylene into the ethylene distributor column, and withdrawing therefrom an ethylene distributor overhead stream comprising hydrogen, methane and ethylene and essentially free of ethane, and an ethylene distributor bottoms stream comprising ethylene and ethane; (iv) directing a major fraction of the ethylene distributor overhead stream into an integrated chill train and existing demethanizer column, and withdrawing therefrom a demethanizer overhead stream comprising hydrogen and methane and essentially free of ethylene, and a demethanizer bottoms stream comprising ethylene substantially free of methane;

(v) directing at least a portion of the stream having a low content of acetylene into the existing splitter column; and

(vi) withdrawing two purified ethylene products, one from the bottom portion of the existing demethanizer column and another from the upper portion of the existing splitter column.

13. The method of claim 12 wherein the total mass flow of the two purified ethylene products is at least 30 percent greater than the mass flow of the purified ethylene product of the existing olefins production facility.

14. An olefins production facility for recovery of at least a purified ethylene product from a gaseous feed mixture comprising methane, ethylene, ethane, propylene, and components heavier than propylene, which facility comprises:

(14-a) a distributive battery comprising separation units which during operation exhibit the ability to separate, from the gaseous mixture a primary effluent stream, comprising about 15 to 90 percent of the ethylene in the feed mixture and methane, essentially free of ethane; and one or more secondary streams, comprising about 10 to 85 percent of the ethylene in the feed mixture and ethane, essentially free of methane; (14-b) a first fractionation group which during operation exhibits the ability to separate from the stream comprising ethylene and methane a first purified ethylene product essentially free of methane; (14-c) a second fractionation group which during operation exhibits the ability to separate from one or more of the streams comprising ethylene and ethane a second purified ethylene product essentially free of ethane.

15. The olefins production facility according to claim 14 wherein the distributive battery of separation units comprises:

(15-a) a first distillation unit which during operation exhibits the ability to separate from the gaseous mixture a first overhead stream, comprising methane, ethylene, ethane, and propylene, and a first bottom stream, comprising components heavier than propylene; (15-b) a second distillation unit which during operation exhibits the ability to separate from the first overhead stream a second overhead stream, comprising ethane and ethylene essentially free of propylene; and second bottom stream, comprising propylene and ethylene, essentially free of methane;

(15-c) a third distillation unit which during operation exhibits the ability to separate from the second overhead stream a third overhead stream, comprising ethylene, essentially free of ethane, and a third bottom stream, comprising ethylene, essentially free of methane; and (15-d) a fourth distillation unit which during operation exhibits the ability to separate from the second bottom stream a fourth overhead stream, comprising ethylene and ethane, essentially free of propylene, and a fourth bottom stream, comprising propylene, essentially free of ethane.

16. The olefins production facility according to claim 14 wherein the first fractionation group comprises an integrated chill train and distillation unit which during operation exhibit the ability to separate from the primary effluent stream comprising ethylene and methane, a first purified ethylene product essentially free of methane.

17. The improved olefins production facility according to claim 14 wherein the distributive battery of separation units comprises:

(17-a) a distillation unit which during operation exhibits the ability to separate, from the gaseous mixture, an overhead stream, comprising methane, ethylene and ethane, essentially free of propylene; and (17-by a subsequent distillation unit which during operation exhibits the ability to separate from the propylene-free overhead stream of (17-a) another overhead stream, comprising about 15 to 90 percent of the ethylene in the propylene-free overhead stream, essentially free of ethane, and a bottom stream, comprising about 10 to 85 percent of the ethylene, essentially free of methane.